Add commit hook and make the verification customizable.

There are three hooks:

    - 'pre-commit' is given an opportunity to inspect what is
      being committed, before we invoke the EDITOR for the
      commit message;

    - 'commit-msg' is invoked on the commit log message after
      the user prepares it;

    - 'post-commit' is run after a successful commit is made.

The first two can interfere to stop the commit.  The last one is
for after-the-fact notification.

The earlier built-in commit checker is now moved to pre-commit.

Signed-off-by: Junio C Hamano <junkio@cox.net>

templates/hooks--post-commit

@@ -0,0 +1,8 @@
+#!/bin/sh
+#
+# An example hook script that is called after a successful
+# commit is made.
+#
+# To enable this hook, make this file executable.
+
+: Nothing